The invention relates to a method and a device for facilitating the insertion of a coiled tube into an oil or gas well, and for applying of impact energy to stuck objects in an oil or gas well.
On inserting a coiled tube into an oil or gas well, in the following referred to as a well, the length of insertion is limited by friction between the coiled tube and the wall of the well. Even if the coiled tube is straightened in a separate straightening apparatus before being introduced into the well, it will adopt the form of a wave or a helix in the well. As the coiled tube is being pushed further and further down the well, and there are more points of contact between the coiled tube and the wall of the well, the total friction increases to a level at which the end of the coiled tube does not proceed further into the well. Further supply of coiled tube only leads to more turns being formed in the helix adopted by the coiled tube.
As is quite natural, the problem arises especially in wells of long horizontal stretches, in which weights at the end of the coiled tube will not contribute to stretching out the coiled tube.
It is known to mount a remotely controlled, motor driven propulsion device, a well tractor, at the end of the coiled tube to draw the coiled tube into the well. A well tractor is expensive and complex, and operational disturbances may easily occur. Furthermore, it is difficult to construct well tractors which are able to proceed and provide sufficient force in wells of small cross-sections. The cross-section is always smallest at the innermost/downmost part of a well, and long wells may also have the smallest cross-sections.
Objects that are stuck in a well, are most commonly loosened by applying impact energy to them. An impact tool which has been arranged to a drill string or a coiled tube, is inserted down to the stuck object and is activated. Known impact tools use a pre-tensioned spring which accelerates a mass, a hammer, which after having achieved appropriate speed, strikes against a stop transferring impact energy to the stuck object. Before each stroke the spring is tensioned by means of a hydraulic mechanism which is activated by a pressure liquid in the drill string or the coiled tube. The spring energy is released when the pre-tensioning has reached a predetermined value. A drawback of this known solution is that very powerful and space-consuming springs have to be provided to achieve the required impact energy. Another known type of impact tool is periodically extended and lifts the drill string or coiled tube which is above the impact tool, and then lets the drill string or coiled tube drop again, so that the mass of the drill string or the coiled tube causes a hammer effect. This type of impact tool has the unfavourable effect that impacts are transferred to the hole drill string or coiled tube in such a way that the couplings and other equipment arranged thereto, may be damaged.
The object of the invention is to provide a method and a simple, inexpensive device for facilitating the insertion of a coiled tube into a well, and for applying impact energy to objects which are stuck in a well. The aim is reached through features as indicated in the following description and subsequent claims.
According to the invention the aim is reached through applying impact changes or pressure strokes to a liquid flowing through a coiled tube or drill string. A pressure stroke in a coiled tube will contribute to briefly overcoming frictional forces between a coiled tube and the wall of the well, so that the coiled tube may be introduced a little further into the well by each pressure stroke.
Pressure strokes may be transferred to a stuck object by the coiled tube or drill string in a known manner being lead into contact with, and possibly attached to, the stuck object. Pressure strokes may also be used to accelerate a mass, a hammer, which in a manner known in itself, strikes against a stop which transfers impact energy to the stuck object.
Pressure strokes is achieved, according to the invention, by periodical shut-off of a liquid flow in the coiled tube or drill string, a valve device being located at or near the outlet of the coiled spring. The valve device may advantageously be such, that it is activated once the liquid flow exceeds a predetermined flow rate. Then it is possible to carry out ordinary well operations by a lower and normal flow rate, and if a need for pressure changes arises, the flow rate is increased to activate the valve device.
To achieve the best possible effect, the valve device should be such, that after having shut off, it remains shut long enough for the pressure rise to spread in the liquid, and so that after having opened, it remains open long enough to re-establish full flow rate.